Busy people all make the same mistake: they assume they are short on time, which of course they are. But time is not their only scarce resource. They are also short on bandwidth. By bandwidth I mean basic cognitive resources — psychologists call them working memory and executive control — that we use in nearly every activity. Bandwidth is what allows us to reason, to focus, to learn new ideas, to make creative leaps and to resist our immediate impulses. We use bandwidth to be a good participant at an important meeting, to be a good boss to an employee who frustrates us and to be attentive parent or spouse.

When we schedule things, we don’t want to just show up, we want to be effective when we get there. This means we need to manage bandwidth and not just manage time. And this is where things get tricky, because bandwidth does not behave the way time does. Time can be dissected easily: an hour can be cut up in many ways. Fifteen minutes on this memo, a five-minute walk to another meeting, 30 minutes at that meeting and then 10 minutes debriefing. Oh, and maybe a quick phone call on the walk to that meeting. The busy are expert at dissection: that’s how they make it all fit.

But bandwidth cannot be dissected like time can. Picture yourself at dinner with a friend whose marriage is on the rocks and wants some advice. Now imagine her request comes at a time when you have a big-project deadline looming. You value her friendship so you make time for dinner, but once you’re there, you find your mind wandering back to that project. You hear the advice you’re giving and feel it’s muddied. You try to console her, but it feels a bit off-key: after all you’ve heard only 70% of what she’s said. The problem of course is that while you’ve made time for her, you didn’t make bandwidth for her.

This is the big mistake: we focus on managing time and end up mismanaging bandwidth. Here is another example. An important strategy memo requires two hours to write. As a good time manager, you find the time for it — a one-hour block between two meetings and two half-hour blocks later. But by the time you’re really focused and have got the previous meeting off your mind, your first hour is nearly over. And the other half hours may as well not have been there. Your two hours got you maybe 30 minutes of quality work.

But we can become better bandwidth managers. First, recognize that different tasks require more or less bandwidth. That round-table project update meeting may be time consuming but not bandwidth consuming. The final decision on what to do about that nice but underperforming employee is not time consuming but is bandwidth consuming. Being a good parent or spouse may be both time and bandwidth consuming.

Second, recognize that some tasks tax your bandwidth even when you are not working on them — a looming deadline or a challenging decision call your mind away from whatever you’re working on. They leave you with less bandwidth for everything else. Finally, other tasks do not tax bandwidth but refresh it. It may be time with family, watching a basketball game, time at the gym or simply doing nothing.

These simple ideas can change how you schedule your day. Don’t place tasks requiring heavy bandwidth (that strategy memo) right after tasks that tax bandwidth. Give yourself more time than you need to get the previous meeting off your mind. For bandwidth-demanding tasks, recognize that big contiguous blocks are better than smaller blocks. Finally, find those tasks that refresh bandwidth and make time for them.

A friend of mine, a busy entrepreneur, confessed to me that he fills up his calendar with fictitious meetings for Tuesday. Sometimes he wanders record shops. (Yes, they still exist!) Sometimes he runs errands that have piled up. Sometimes he just loafs on the sofa. These are investments in bandwidth. “I’m just more productive overall since I started secret Tuesdays,” he told me. That’s the real secret of secret Tuesdays.